Electronic pressure transducers are commonly used by medical personnel to monitor blood pressure in the cardiovascular system and the pressure of other physiological fluids. Such devices are typically discarded after being used with a single patient, thereby avoiding any risk of cross contamination of patients. Although the cost of the pressure transducer package that is used in disposable devices of this type is relatively low, it is the most significant portion of the overall cost. Accordingly, it is desirable to substantially reduce the cost of such devices by isolating the pressure transducer from contact with the physiological fluids of a patient so that the pressure transducer portion of the device can be reused with other patients without risk of cross contamination.
U.S. Pat. No. 4,920,972 discloses a reusable pressure transducer that is used in connection with a disposable portion for monitoring blood pressure through a fluid line in which a saline solution or other medicinal fluid flows into a patient's cardiovascular system. The disposable portion has a dome that covers a cavity. When monitoring cardiovascular pressure, an input port to the cavity in the disposable portion is coupled to a source of the medicinal fluid, and an output port of the disposable portion is coupled through the line to a patient's blood vessel. The cavity is covered with a rubber diaphragm that is sealed around the periphery of the cavity. Pressure in the medicinal fluid flowing through the disposable portion is indicative of the blood pressure in the patient's blood vessel and is applied to the rubber diaphragm covering the cavity.
The reusable portion includes a similarly sized cavity, also covered with a rubber diaphragm, which is in fluid communication with a pressure transducer mounted in the body of the reusable portion. This patent discloses both an earlier embodiment, in which the cavity in the reusable portion is filled with an oil, and an improved embodiment in which the cavity in the reusable portion is filled with a gel. A slight distention of the rubber diaphragm in the reusable portion occurs as the gel, which is formed in place from an injected liquid, cures. The slightly distended robber diaphragm of the reusable portion thus has a "good fit" with the diaphragm of the disposable portion, when the disposable portion and reusable portion are coupled together with their respective rubber diaphragms in contact. Pressure exerted by the rubber diaphragm in the disposable portion against the rubber diaphragm of the reusable portion is transmitted through the gel (or oil) to the pressure transducer.
In U.S. Pat. No. 3,720,201, a disposable body fluid pressure monitor is disclosed that includes a tube coupled in fluid communication with a patient's body fluid. A diaphragm within the tube separates the body fluid from a cavity that is in fluid communication with a pressure sensing aneroid manometer or other standard pressure sensor. The tube is discarded after being used with a patient, but the pressure sensing device is reused, since it is protected from contamination by the diaphragm. A similar arrangement is disclosed in U.S. Pat. No. 3,890,842.
Although the pressure monitoring device disclosed in the above noted U.S. Pat. No. 4,920,972 has several clear advantages over other prior art reusable pressure monitoring devices, it does not achieve a desired high frequency response. The volume of gel (or oil) within the cavity of the reusable portion of this prior art device tends to impose an upper limit on the frequency response by providing additional compliance to the device.
Ideally, a physician will want to observe certain aspects of blood pressure that can only be determined with a higher frequency response than is provided by the prior art device discussed above. For example, assuming that the line coupling the disposable portion of the pressure monitoring device to the patient's blood stream is about 72 inches in length, the pressure monitoring device should have a maximum frequency response of at least 20 Hz. With an upper frequency response at this level, the pressure monitoring device will enable a physician to observe the dichrotic notch in the signal produced by the pressure transducer. The dichrotic notch in the blood pressure signal corresponds to the opening and closing of the aortic valve in the patient's heart. By observing the dichrotic notch, a physician can evaluate the quality of the patient's heart beat and the quality of the systolic and diastolic pressure measurements. A sufficiently high frequency response also enables medical personnel to detect the presence of bubbles in the fluid line, between the patient's blood vessel and the disposable portion of the pressure monitoring device.
The volume of fluid (including a gel) that couples the diaphragm of the reusable portion of the pressure monitor to the pressure transducer affects the compliance of the reusable portion, which directly affects the frequency response of the pressure monitor. Due to variations in the volume of fluid in the cavity below the diaphragm of the reusable portion caused by temperature, it is not sufficient to simply provide a small closed cavity. Instead, it is preferable to provide for variations in the volume of this fluid, for example, by incorporating a reservoir for the fluid in the reusable portion. However, the added volume of fluid in a reservoir and its compliance can decrease the maximum frequency response of the pressure monitoring device. The prior art does not address this problem.